In the fabrication of semiconductor wafers, one recurring problem has been temperature measurement and control. When processing a semiconductor substrate, the temperature of the two sides of the substrate being processed is of critical importance. Overheating can cause dopants to permeate subjacent layers, and under-heating can produces layers which are unreliable and subject to lower tolerances. In order to promote desired photo assisted and chemical reactions and suppress unwanted phenomena, several solutions have been put forth to gauge the substrate's temperature to avoid overheating and under-heating, and thus provide optimized ramp up, steady state and/or ramp down cycle. While the temperature measurement techniques lack precision, wafer temperature non-uniformity during transient Rapid Thermal Processing (RTP) and plasma processing also are issues of concern.
The known solutions to control wafer temperature non-uniformity include rotating the substrate during heating, adjusting different lamps individually, or having the lamps in different zones.
FIGS. 1(a) and (b) illustrate the temperature variation of a semiconductor wafer undergoing during thermal processing. FIG. 1(a) demonstrates the heat radiation, generated from heat lamp 11 which reflects off of reflector lens 12 onto a wafer substrate 13. Rings 14 and 15 at the edges of substrate 13 correlate to the heat absorbing curve depicted in FIG. 1(b).
As FIG. 1(b) shows, the temperature measured at the edge of the wafer (rings 14 and 15) is either increasing or decreasing in temperature and therefore the temperature is not evenly distributed across from the inner portion 16 to the edge of wafer substrate 13. Moreover, the temperature uniformity for patterned wafers, wafers with different films having different thermal expansion coefficients, are going to be even more difficult to control as far as the temperature issue is concerned. Non-uniformity becomes even a bigger factor when the wafer size is larger, thus allowing the disadvantage of, greater heat edge loss at the wafer's edge, and greater temperature variation from the center of the wafer to its edge.
It is therefore desired, during an RTP process, to heat a semiconductor wafer in such a manner as to provide for the wafer's edges to be heated and maintained at a uniform temperature that is substantially the same as the temperature distributed across the inner wafer portion.